Formation of compact and GaiaBH-like binaries in young open star clusters: Early Evolution

Abstract

Gaia’s third data release found three wide, dormant black hole binaries: GaiaBH1, GaiaBH2, and GaiaBH3. isolated binary evolution models don’t reproduce similar orbital properties. Most simulated Milky Ways yield nearly zero detectable systems like these. Dynamical formation in young star clusters does better, producing Gaia-like binaries up to a thousand times more efficiently per unit of stellar mass. This thesis explores what this formation channel will yield in a realistic environment, using N-body simulations of open clusters that undergo instantaneous gas expulsion. We ran 81 star cluster simulations. The pipeline combines MCLUSTER for initial mass and binary distributions, the AGAMA library for phase-space sampling in a residual gas potential, and PeTar with SSE/BSE stellar evolution. Every cluster has a star formation efficiency of SFE = 0.17, a primordial binary fraction of fb = 0.5, and solar metallicity (Z = 0.02). They sit in a Milky Way-like tidal field at roughly 8 kpc from the Galactic center. Initial mass and phase-space random seeds are the only things that vary between runs. Each cluster evolved for 150 Myr, with snapshots saved every 1 Myr. We tracked black hole–star, neutron star-star, and black hole–black hole binaries, classifying each as primordial, dynamical, retained, or escaped. Across the 81 runs, the simulations produced 4006 binary systems, mostly primordial, including 271 black hole–star and 144 black hole–black hole pairs. Black hole–luminous companion binaries tend toward orbital periods of 170 − 200 days, near-zero eccentricities, and companion masses of 1.0 − 1.2 M⊙. These are within Gaia’s detectability window but still don’t match the observed systems well enough. The bottleneck is the mass-ratio distribution (q ∈ [0.1, 1]), which doesn’t generate the extreme mass-ratio progenitors (q ∼ 0.03 − 0.05) that Gaia-like systems require. Getting there means trying different pairing prescriptions, exploring lower metallicities, and running bigger simulations.